Electrochromism is a phenomenon that oxidation and reduction reactions are reversely carried out to reversely change a color, as voltage is applied. A device utilizing this electrochromism is an electrochromic device. Various researches have been conducted on an electrochromic device to realize an application derived from characteristics of electrochromism.
As for electrochromic materials used for electrochromic devices, there are organic materials, and inorganic materials. The organic materials can be colored in various colors depending on the molecular structures thereof, and therefore the organic materials are promising materials to be used in color display devices. On the other hand, the inorganic materials have a problem in controlling a color. Using this characteristic, use of the inorganic materials for applications, in which a low color saturation is an advantage, such as a dimming glass, or a neutral density (ND) filter, has been considered.
The aforementioned electrochromic device typically has a structure where an electrochromic material is formed between two facing electrodes, and the space between the electrodes is filled with an electrolyte capable of ion conduction. In this structure, oxidation and reduction reactions are carried out. Electrochromism is an electrochemical phenomenon, hence a performance (ion conductivity) of the electrolyte layer affects a response speed or a memory effect of color. In the case where the electrolyte layer is a liquid state, in which an electrolyte is dissolved in a solvent, a quick response speed tends to be attained. However, an improvement thereof by solidification or gelation has been studied in view of strength of an element, and reliability. Specifically, an electrolytic solution has been conventionally used in a battery as an electrochemical element, or an electrochromic device. Therefore, the electrolytic solution is leaked, and the contents inside the battery are dried due to evaporation of the solvent. Inside the battery container, moreover, part of the barrier is dried due to deviation of the electrolytic solution. These factors may cause an increase in internal impedance, and internal short circuit.
Especially in the case where the electrochromic device is used as a dimming glass, or a display, at least one side of the electrochromic device needs to be sealed with a transparent material, such as a glass, or a plastic. Therefore, it is difficult to completely seal the electrolyte with a metal or the like, hence leak or evaporation of the electrolytic solution is a severe problem.
As for a method for solving the aforementioned problems, use of a solid polymer electrolyte is proposed. Examples of the solid polymer electrolyte include a solid solution of a matrix polymer containing an oxyethylene chain or an oxypropylene chain, and an inorganic salt. These solid solutions are complete solids, and they have excellent processability, but having a low electric conductivity. In order to improve electric conductivity of the solid polymer electrolyte, therefore, proposed are a method where an organic electrolytic solution is dissolved in a polymer to form into a semi-solid, and a method where a liquid monomer to which an electrolyte is added, and the liquid monomer is reacted through a polymerization reaction to thereby form a crosslinked polymer containing the electrolyte. However, these proposed methods have not yet reached the level of practical use.
Meanwhile, the electrochromic device is typically produced by forming an electrochromic material between two electrodes facing each other, followed by bonding via an electrolyte layer capable of ion conduction. Therefore, an application of a flat shape can be easily formed, but there is a problem that the electrochromic device cannot be easily applied for an application of a curved shape, or a three-dimensional (3D) shape. If the electrochromic device can be applied for a three-dimensional (3D) shape, such as a lens, a range of the applications thereof is widened as an optical use. However, there is a problem that an optical failure tends to occur due to precision of a curve or positioning precision of two substrate to be bonded. In case of spectacle lenses, the spectacle power of the lenses needs to be adjusted depending on a user, and therefore the necessity of preparing many substrates having different curves becomes a problem in mass-production.
In order to solve the aforementioned problems, there has been an attempt to produce an electrochromic device without a bonding process. In the conventional art, however, it is difficult to form the aforementioned electrochromic device on a support by a thin film forming process. In the case where an electrode layer is formed on an electrolyte layer in order to omit the bonding process, use of an all solid electrolyte causes the aforementioned problem that a response speed is slow. If an organic material layer is used as an all solid electrolyte layer, moreover, electric resistance of an electrode layer to be formed on an electrolyte layer tends to be high, and the electrochromic device cannot be regularly driven with oxidation reduction. Particularly when an oxide layer (e.g., ITO, IZO, SnO2, AZO, and GZO) formed by vacuum film formation, which is typically adapted as a transparent electrode, is formed on a surface of an organic film, there is a problem that it is difficult to achieve transparency and electric conductivity at the same time.
In the case where an inorganic material layer is used as an all solid electrolyte layer, on the other hand, an electrochromic compound for use is limited to an inorganic electrochromic compound. Example of an electrochromic device using the inorganic electrochromic compound include an electrochromic device having a structure where a reduction coloring layer and an oxidation coloring layer are provided to face each other with a solid electrolyte layer being between them. Proposed is an electrochromic device, which has the reduction coloring layer composed of a material containing tungsten oxide and titanium oxide, the oxidation coloring layer composed of a material containing nickel oxide, and a transparent intermediate layer, which is disposed between the oxidation coloring layer, and a solid electrolyte layer, and contains metal oxide other than nickel oxide, or a metal, or a complex containing metal oxide other than nickel oxide and a metal as a main component (see, for example, PTL 1). This literature discloses that repetition properties and response are improved by forming the intermediate layer, hence coloring-discharging can be performed within a few seconds. However, the electrochromic device disclosed in PTL 1 has a complicated structure, and cannot be easily increased in its size, as multiple layers of the inorganic compound layer are formed by vacuum film formation, as well as increasing a cost. Moreover, it cannot avoid an influence from heat during the film formation process, and therefore the substrate for use is limited to a heat resistant material, such as glass. Furthermore, the inorganic electrochromic reaction is easily influenced by moisture, and a color tone of the inorganic electrochromic compound is limited to a tone of blue.